Laccases are abundant in microorganisms specialized in lignocellulose degradation, but the exact role of the enzymes in this process remains poorly understood. Laccases catalyze oxidation of lignin, initially producing a phenoxy radical. The heterogeneous lignin structure can stabilize these reactive radicals, but eventually, they dissipate by uncharacterized mechanisms. We show that the laccase-catalyzed oxidation of lignin leads to formation of H2O2, likely via a reaction between lignin radicals and O2. Up to 25 μM of H2O2 was generated by each of four fungal and bacterial laccases catalyzing oxidation of both organosolv lignin and birch wood. The generated H2O2 activated lytic polysaccharide monooxygenases (LPMOs) both in separate reactions, where the LPMOs were fed with laccase-generated H2O2, and in one-pot reactions, where laccases and LPMOs acted simultaneously. These observations are new, and our results reveal a potentially critical connection between conversion of lignin and polysaccharides, suggesting that laccases generate H2O2 for other lignocellulose-depolymerizing enzymes such as lignin peroxidases and LPMOs. This new role for laccases may be exploited in application settings.